1. Field of the Invention
The present invention relates to a ferroelectric thin film containing lead (Pb), a method of producing said thin film and a coating solution for making the ferroelectric thin film.
2. Description of the Related Art
A ferroelectric thin film has received attention in recent years, since it is expected that the ferroelectric thin film can be applied to the production of semiconductor memory devices and the like. (For example, attention is directed to literature I: Ceramics Vol. 30, No. 6, pp. 499-507 (1995)). In this respect, studies on a ferroelectric thin film of a composition of PbZr.sub.x Ti.sub.1-x O.sub.3 (0&lt;x&lt;1, hereinafter referred to as "PZT") have been particularly carried out. For making the PZT thin film, several methods, such as a sputtering method, a CVD method, a wet method or the like, have been tried. Since the wet method does not use a vacuum system, it is superior in the points that it can reduce the cost of making the thin film and mass-producing it and it is capable of producing a thin film on a large area. According to the wet method, a coating solution for making the ferroelectric thin film is applied on a substrate and then is baked to produce the desired thin film. A coating solution for making the PZT thin film is manufactured by a so-called sol-gel method in many cases.
There is a problem, however, in that a large leakage current tends to flow through the prior art PZT thin film. In particular, there is a problem in the thinner film thickness range (i.e. in a range of not more than 0.3 .mu.m) in that the dielectric withstand voltage of the film deteriorates in addition to the increased leakage current. These problems are caused by the lead (Pb) loss in the thin film. Moreover, the PZT film made by the wet method in particular tends to possess these problems. This is thought to be due to the occurrence of lead loss in the baking process. To solve this problem, there has been proposed a method of adding excessive lead in advance in amounts equivalent to the loss (a few %). But even in this case, the thin film is in such a state that it is deficient in lead in close proximity to the surface thereof. Further, if lead is added in excessive amounts to avoid this state of lead deficiency, the electrical conductivity of the film is increased; thus, the leakage current is increased. It is thought that not only PZT thin films but also other various kinds of ferroelectric thin films containing Pb will produce this problem.
Accordingly, it is desirable to produce a ferroelectric thin film containing Pb and with a smaller leakage current than was previously possible and a process of making such film, as well as a coating solution for producing the thin film.